Pörschke D
Nucleic Acids Res. 1979 Mar;6(3):883-98. doi: 10.1093/nar/6.3.883.
Large changes of UV absorbance and CD spectra as well as specific relaxation processes with time constrants around 50 mus are found for the association of Mg++ with A(pa)n. The Mg++ binding constants strongly increase with increasing n. The relaxation data demonstrate that a large fraction of Mg++ bound to short A(pA)n forms inner sphere complexes (ISC), with H2O molecules from the inner hydration sphere of Mg++ exchanged against some site (s) of the oligomer. This fraction decreases from about 85% for A(pA)4 to less than 10% for A(pA)17. A parallel decrease is observed in the relative change of CD spectrum upon Mg++ binding from 77.5% for A(pA)4 to 13.4% for (pA)17. The rate of ISC formation decreases with increasing n suggesting some (probably sterical) hindrance effect at high n. The data support the conclusion that Mg++ favours the formation of outer sphere complexes with linear polynucleotides and require a special chain folding for ISC. Measurements of Mg++ binding to C(pC)5, U(pU)5, I(pI)5 and d[A(pA)5] did not give evidence for the formation of ISC, indicating that both specific base and sugar residues are required for ISC. These results suggest the possibility that Mg++ISC ARE USED FOR SPECific recognition of nucleic acid sequences.
发现Mg++与A(pa)n缔合时，紫外吸光度和圆二色光谱有很大变化，以及在约50微秒的时间常数下有特定的弛豫过程。Mg++结合常数随n的增加而强烈增加。弛豫数据表明，与短A(pA)n结合的大部分Mg++形成内球络合物（ISC），Mg++内水合球中的水分子与寡聚物的某些位点发生交换。这一比例从A(pA)4的约85%降至A(pA)17的不到10%。Mg++结合后圆二色光谱的相对变化也有类似的下降，从A(pA)4的77.5%降至(pA)17的13.4%。ISC形成速率随n的增加而降低，表明在高n时存在某种（可能是空间位阻）阻碍效应。这些数据支持以下结论：Mg++有利于与线性多核苷酸形成外球络合物，而ISC需要特殊的链折叠。对Mg++与C(pC)5、U(pU)5、I(pI)5和d[A(pA)5]结合的测量未提供形成ISC的证据，表明ISC需要特定的碱基和糖残基。这些结果表明Mg++ISC用于核酸序列特异性识别的可能性。
